1. Field of the Invention
The present invention relates to the papermaking arts. More specifically, the present invention is a papermakers' fabric of the on-machine-seamable (OMS.RTM.) variety, such as an OMS.RTM. press fabric for the press section of a papermachine.
2. Description of the Prior Art
During the papermaking process, a fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, on a moving forming fabric in the forming section of a papermachine. A large amount of water is drained from the slurry through the forming fabric during this process, leaving the fibrous web on the surface of the forming fabric.
The newly formed web proceeds from the forming section to a press section, which includes a series of press nips. The fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two press fabrics. In the press nips, the fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the fibers in the web to one another to turn the fibrous web into a sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the web.
The web finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The web, or newly formed paper sheet, itself is directed in a sinuous path sequentially around each in the series of drums by a dryer fabric, which holds the web closely against the surfaces of the drums. The heated drums reduce the water content of the web to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the papermachine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speed. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
Referring, for the moment, specifically to press fabrics, it should be recalled that, at one time, press fabrics were supplied only in endless form. This is because a newly formed paper sheet is extremely susceptible to marking in the press nip by any nonuniformity in the press fabric or fabrics. An endless, seamless fabric, such as one produced by the process known as endless weaving, has a uniform structure in both its longitudinal (machine) and transverse (cross-machine) directions. A seam, such as a seam which may be used to close the press fabric into endless form during installation on a papermachine, represents a discontinuity in the uniform structure of the press fabric. The use of a seam, then, greatly increases the likelihood that the paper sheet will be marked in the press nip.
In brief, the seam region of any workable on-machine-seamable (OMS.RTM.) press fabric must behave under load, that is, under compression in the press nip or nips, like the rest of the press fabric, and must have the same permeability to water and to air as the rest of the press fabric, in order to prevent the periodic marking of the paper product being manufactured by the seam region. OMSO.RTM. is a registered trademark of Albany International Corp.
Despite the considerable technical obstacles presented by these requirements, it remained highly desirable to develop an on-machine-seamable (OMS.RTM.) press fabric, because of the comparative ease and safety with which it could be installed on the press section. Ultimately, these obstacles were overcome with the development of press fabrics having seams formed by providing seaming loops on the crosswise edges of the two ends of the fabric. The seaming loops themselves are formed by the machine-direction (MD) yarns of the fabric. A so-called "pin-seam" is formed by bringing the two ends of the press fabric together, by interdigitating the seaming loops at the two ends of the fabric, and by directing a so-called pin, or pintle, through the passage defined by the interdigitated seaming loops to lock the two ends of the fabric together. Needless to say, it is much easier and far less time-consuming to install an OMS.RTM. press fabric, than it is to install an endless press fabric, on a papermachine.
One method to produce a press fabric that can be joined on the papermachine with a "pin seam" is to flat-weave the fabric. In this case, the warp yarns are the machine-direction (MD) yarns of the press fabric. To form the seaming loops, the warp ends are woven some distance back into the fabric body in a direction parallel to the warp yarns. Another technique, far more preferable, is a modified form of endless weaving, which normally is used to produce an endless loop of fabric. In modified endless weaving, the weft, or filling, yarns are continuously woven back and forth across the loom, in each passage forming a loop on one of the edges of the fabric being woven by passing around a loop-forming pin. As the weft yarn, or filling yarn, which ultimately becomes the MD yarn in the press fabric, is continuous, the seaming loops obtained in this manner are stronger than any that can be produced by weaving the warp ends back into the ends of a flat-woven fabric.
Originally, single monofilament strands were used in both the machine and cross-machine directions of OMS.RTM. press fabrics. The relative stiffness of monofilament ensures that it will have the requisite good seaming-loop formation properties. Experience showed, however, that single monofilament strands are difficult to weave and have insufficient elasticity in the machine direction for many kinds of contemporary presses. Tensile failure and seam breakage were frequently observed.
Another difficulty is presented by the very open, rigid, incompressible structure of base fabrics woven from single monofilament. For some papermaking applications, this incompressibility is not a problem, and may even be ideal. However, for positions that have poor auxiliary fabric dewatering capacity, or produce mark-sensitive paper grades, a softer, more compressible base fabric is needed.
A more compressible base fabric may be obtained by weaving with multifilament or plied monofilament yarns, instead of with single monofilament strands. However, yarns of these types do not have the rigidity necessary for good loop formation or for maintaining the integrity of the seam area during loop meshing when the seam is to be closed. Moreover, because yarns of these types are twisted, loops formed from them tend to rotate about axes lying in the planes of the loops. When this rotation, known as the secondary helix effect, occurs, it causes the loops to depart from the ideal orientation needed to form the pin seam. Such departure makes it difficult, if not impossible, to properly interdigitate the loops at each end of the press fabric during closure, as well as to direct the pintle through the passage defined by the interdigitated loops.
Various attempts have been made in the prior art to overcome these difficulties by making the loop-forming MD yarns act like monofilament. In U.S. Pat. No. 5,005,610, the MD yarns in an OMS.RTM. Papermakers' fabric have a composite structure including braided monofilament strands. The braided yarn forms seaming loops which resist deformation and, because they are balanced with regard to twist, form seaming loops which are not susceptible to "secondary helix effects" rotation from the ideal plane geometry of the seam.
In U.S. Pat. No. 5,204,150, the MD yarns in an OMS.RTM. papermakers' fabric are plied/twisted yarns extruded from a resin which partially melts during the heat-setting of the fabric, giving the MD yarns a monofilament-like character. Even though not balanced due to the twising and plying, the fusion caused by the partial melting of the individual ends prevents loop rotation from the ideal seam geometry.
Finally, in U.S. Pat. No. 5,391,419, the MD yarns of a pin-seamable papermakers' fabric are plied/twisted yarns having a coating which gives the yarn a monofilament-like structure. The coating may be either permanent, semi-permanent or soluble. Even though the yarns may not be balanced, the coating prevents loop rotation.
The present invention is a different approach for providing an OMS.RTM. papermakers' fabric having plied/twisted MD yarns with monofilament-like seaming loops.